1. Field of the Invention
The present invention relates to an optical fiber transmission line employable as an optical transmission line in a wavelength division multiplexing (WDM) transmission system, and an optical cable including the same.
2. Related Background Art
A WDM transmission system enables high-speed, large-capacity optical communications by utilizing a plurality of signal channels. The optical transmission line laid in each repeating section of the WDM transmission system is desired to have an excellent transmission characteristic in a signal wavelength region (e.g., 1.55-xcexcm wavelength band). Therefore, as an optical transmission line having an excellent signal transmission characteristic, optical fibers whose transmission characteristic changes along the longitudinal direction thereof have been proposed.
For example, a conventional optical fiber transmission line described in T. Naito, et al., xe2x80x9c1 Terabit/s WDM Transmission over 10,000 km,xe2x80x9d ECOC 99, PD2-1 (1999) (first literature) is constituted by a single-mode optical fiber (an optical fiber having a positive chromatic dispersion) positioned on the upstream side in the signal-advancing direction and a dispersion-compensating optical fiber (an optical fiber having a negative chromatic dispersion) positioned on the downstream side. In the 1.55-xcexcm wavelength band, the single-mode optical fiber has a positive chromatic dispersion and a relatively large mode field diameter. On the other hand, the dispersion-compensating optical fiber has a negative chromatic dispersion and a relatively small mode field diameter in the 1.55-xcexcm wavelength band, and is likely to generate nonlinear optical phenomena in general.
In the conventional optical fiber transmission line described in the above-mentioned first literature, signals successively propagate through the single-mode optical fiber and dispersion-compensating optical fiber. Though the signals propagating through the single-mode optical fiber have a high power, nonlinear optical phenomena are restrained from occurring since the single-mode optical fiber has a relatively large mode field diameter. The signals lower their power while propagating through the single-mode optical fiber, and the signals having lowered their power reaches the dispersion-compensating optical fiber. As a consequence, the occurrence of nonlinear optical phenomena is sufficiently suppressed even when the signals propagate through the dispersion-compensating optical fiber having a relatively small mode field diameter. Further, since the single-mode optical fiber and dispersion-compensating optical fiber have chromatic dispersions different from each other, the cumulative chromatic dispersion of the optical fiber transmission line as a whole will be kept low if the ratio of their lengths is designed appropriately. Thus, an optical fiber transmission line in which a single-mode optical fiber and a dispersion compensating optical fiber are successively disposed along the signal-advancing direction can effectively restrain the transmission quality from deteriorating due to nonlinear optical phenomena and cumulative chromatic dispersion.
On the other hand, the optical fiber transmission line disclosed in U.S. Pat. No. 5,894,537 (second literature) is a unitary optical fiber in which a plurality of positive dispersion portions having a positive chromatic dispersion and a plurality of negative dispersion portions having a negative chromatic dispersion are alternately arranged adjacent each other along its longitudinal direction. The occurrence of nonlinear optical phenomena, such as four-wave mixing in particular, is suppressed when the absolute value of chromatic dispersion in each of the positive and negative dispersion portions is set greater, and the deterioration in transmission quality caused by cumulative chromatic dispersion is suppressed when the absolute value of mean chromatic dispersion in the optical fiber transmission line as a whole is set lower.
The inventors have studied the conventional techniques mentioned above and, as a result, have found problems as follows. Namely, the conventional optical fiber transmission line described in the first literature is required to change its transmission characteristic along the longitudinal direction while yielding a desirable value of mean transmission characteristic in the optical fiber transmission line as a whole. Consequently, each optical fiber constituting this optical fiber transmission line is restricted in terms of its transmission characteristic or its length. That is, the conventional optical fiber transmission line is required to have a small mean chromatic dispersion as a whole, whereby it is necessary that the ratio of respective lengths of the single-mode optical fiber and dispersion-compensating optical fiber constituting the optical fiber transmission line be set to a predetermined value.
The conventional optical fiber transmission line described in the above-mentioned second literature is also required to have a small mean chromatic dispersion as a whole, whereby it is necessary that the ratio of respective lengths of the positive and negative dispersion portions be set to a predetermined value.
Even when an optical fiber transmission line designed so as to yield a desirable mean transmission characteristic as a whole is made, if an end portion of this optical fiber transmission line is cut off, then the mean transmission characteristic of thus cut optical fiber transmission line as a whole may not attain the desirable value. When the process of making an optical cable from an optical fiber transmission line such as that mentioned above is concerned, for example, both end portions of each optical fiber are cut off until a desirable condition is obtained in each of steps of welding water-pressure-resistant copper tubes, extruding sheaths, and the like in the case of the optical fiber transmission line. The fiber lengths (hereinafter referred to as xe2x80x9ccut lengthsxe2x80x9d) of both end portions cut in such individual steps amount to several hundreds of meters.
Between before and after the cutting, the mean transmission characteristic of the optical fiber transmission line as a whole changes according to the cut length. In a transmission line in which a chromatic dispersion with a large absolute value locally occurs, such as the optical fiber transmission lines described in the above-mentioned first and second literatures in particular, the change in mean transmission characteristic (mean chromatic dispersion) in the optical fiber transmission line as a whole between before and after the cutting is large.
Concerning the optical fiber transmission line described in the above-mentioned first literature, the problem mentioned above will be explained specifically with reference to a case of making a submarine optical cable having a cross-sectional structure shown in FIG. 1A by way of example. Here FIG. 1A is a view showing the cross-sectional structure of the submarine optical cable, whereas FIG. 1B is a view showing the cross-sectional structure of the optical fiber unit included in the submarine optical cable. On the other hand, FIGS. 2A to 2D are views showing respective changes in the length of optical fiber transmission line in individual steps of making the optical cable.
As shown in FIG. 1A, a three-part metal tube 310, a high-tension steel twisted wires 320, a copper tube 330, and an insulating plastic layer 340 are successively disposed on the outer periphery of an optical fiber unit 300 holding a plurality of optical fiber transmission lines, so as to construct the optical cable. Here, as shown in FIG. 1B, the optical fiber unit 300 has a structure in which a plurality of optical fiber transmission lines 410 are secured about a tension member 420 by way of a buffer layer (unit filler resin) 430.
Before bundling, each optical fiber transmission line 410 is, as shown in FIG. 2A, constituted by a single-mode optical fiber 412 (SMF: Single-Mode optical Fiber) and a dispersion-compensating optical fiber 411 (DSF: Dispersion-Compensated optical Fiber) having chromatic dispersions of about 20 ps/nm/km and about xe2x88x9245 ps/nm/km, respectively.
At the time of bundling, a plurality of optical fiber transmission lines 410 whose end portions on the SMF side and the DCF side are cut by 10 m and 80 m, respectively, as shown in FIG. 2B. Then, upon welding water-pressure-resistant copper tubes, the end portions on the SMF side and the DCF side are cut by 90 m and 350 m, respectively (FIG. 2C). Also, upon extruding a sheath, the end portions on the SMF side and the DCF side are cut by 150 m and 70 m, respectively (FIG. 2D). After the foregoing steps, the cut length finally becomes 250 m on the SMF side, whereby the chromatic dispersion changes by 5 ps/nm (=20 ps/nm/kmxc3x970.25 km) due to the end portion cutting. On the DCF side, on the other hand, the cut length finally becomes 500 m, whereby the chromatic dispersion changes by xe2x88x9222.5 ps/nm(=45 ps/nm/kmxc3x970.5 km) due to the end portion cutting. As a consequence, in the case where a plurality of optical fiber transmission lines are bundled so as to make an optical cable as mentioned above, the cumulative chromatic dispersion of each optical fiber transmission line included in the finally obtained optical cable would change by xe2x88x9217.5 ps/nm.
If the cut length in each of both ends of an optical fiber transmission line can be estimated, then each optical fiber transmission line to be cabled may be designed and made while taking account of the amount of change in cumulative chromatic dispersion corresponding to thus estimated cut length. However, the cut length varies depending on whether the conditioning in each step of cable making is fine or not, and further increases (thereby shortening the total length of optical fiber transmission line) if a trouble occurs, whereby the transmission characteristic greatly fluctuates among the resulting optical cables. Therefore, the cut length upon making cables cannot be estimated in practice, and the amount of change in cumulative chromatic dispersion upon cabling cannot be estimated, either.
Meanwhile, the cumulative chromatic dispersion of each optical fiber transmission line is required to match its designed value with an error of about several ps/nm on average in the case of a submarine optical cable. If the cumulative chromatic dispersion changes by several tens of ps/nm due to the end portion cutting as mentioned above, and the amount of change in cumulative chromatic dispersion varies according to uneven cut lengths, however, then it is difficult for the cumulative chromatic dispersion of each optical fiber transmission line included in a single optical cable to match its designed value with an error of about several ps/nm. This is particularly remarkable in the case where a single submarine optical cable corresponds to the whole repeating section, and is similarly remarkable in the case of a ground optical cable. Here, the repeating section refers to any of a section from a transmitting station to a repeater station including an optical amplifier or the like, a section between repeaters, and a section from a repeater station to a receiving station.
In addition, the optical fiber transmission line whose transmission characteristic changes along its longitudinal direction further has a problem as follows. Namely, the optical fiber transmission line described in the above-mentioned first literature has a directivity concerning the advancing direction of signals such that the signals propagate only in one direction from the single-mode optical fiber side to the dispersion-compensating optical fiber side. Since the directivity of the optical fiber transmission line having such a directivity cannot be recognized in appearance, there is a possibility that it cannot be determined from which end portion the signals should be inputted. Though individual fiber transmission lines can be identified if they are painted with respective colors different from each other in an optical cable in which optical fiber transmission lines having such a directivity are bundled, the directivity still cannot be recognized thereby, so that it cannot be determined from which end portion the signals should be inputted, either. Therefore, it is always necessary to grasp whether the end portion of the optical cable in a wound state is on the upstream side (signal-input side) or downstream side (signal-output side) in each step of the manufacturing process and operations for connecting with repeater stations.
In general, inspection,, verification, and the like may be added to the process of making an optical cable according to circumstances. Since rewinding may be effected by a number of times greater than that in a normal manufacturing step as such due to an operational reason, the directivity of optical cable immediately after the making thereof cannot always be determined constant. Consequently, there are cases where a rewinding operation is necessary for rearranging the directivity of optical cable when connecting the optical cable to a repeater station. When the optical cable has a length of 50 km, for example, it takes at least one day to rewind the optical cable, thereby complicating the production control and increasing the operating cost.
In order to overcome the problems mentioned above, it is an object of the present invention to provide an optical fiber transmission line comprising a structure for making extra rewinding operations unnecessary at the time of making a cable, while having a structure for making it possible to maintain a desirable mean transmission characteristic as a whole regardless of the fluctuation in total length accompanying the cutting of end portions; and an optical cable including the same.
The optical fiber transmission line according to the present invention comprises, as a structure suitable for an optical cable, a main transmission line, and first and second end sections connected, respectively, to both ends of the main transmission line. The main transmission line includes an optical fiber whose polarities of chromatic dispersion at a predetermined wavelength alternate along a longitudinal direction thereof. The first end section includes an optical fiber, connected to a first end of the main transmission line, having, at the predetermined wavelength, a chromatic dispersion with an absolute value not greater than that of the mean chromatic dispersion of the main transmission line as a whole. The second end section includes an optical fiber, connected to a second end of the main transmission line opposite from the first end, having, at the predetermined wavelength, a chromatic dispersion with an absolute value not greater than that of the mean chromatic dispersion of the main transmission line as a whole.
The first and second end sections are provided in order to suppress fluctuations of the transmission characteristic of the optical fiber transmission line as a whole in the cable making process, while each of the first and second end sections has a length which is not greater than 5% of the total length of the optical fiber transmission line. The first and second end sections have a maximum degree of influence with an absolute value of 0.2 ps/nm/km or less upon the mean chromatic dispersion of the optical fiber transmission line as a whole, in order to lower the influence of their own fluctuations in length due to cutting upon the optical fiber transmission line as a whole.
In the optical fiber transmission line, the main transmission line may be constituted by a plurality of kinds of optical fibers. In this case, the main transmission line includes one or more first optical fibers having a positive chromatic dispersion at the predetermined wavelength and one or more second optical fibers having a negative chromatic dispersion at the predetermined wavelength, whereas the first and second optical fibers are alternately disposed adjacent each other along the longitudinal direction of the optical fiber transmission line. In the optical fiber transmission line, the main transmission line may include a unitary optical fiber in which one or more first portions (positive dispersion portions) having a positive chromatic dispersion at the predetermined wavelength and one or more second portions (negative dispersion portions) having a negative chromatic dispersion at the predetermined wavelength are alternately disposed adjacent each other along the longitudinal direction thereof. While each case has a locally large chromatic dispersion, the mean chromatic dispersion of the optical fiber transmission line as a whole can be kept low.
Specifically, in the optical fiber transmission line according to the present invention, the first end section has a chromatic dispersion with an absolute value of 5 ps/nm/km or less at the predetermined wavelength, and the second end section also has a chromatic dispersion with an absolute value of 5 ps/nm/km or less at the predetermined wavelength. Since each of the first and second end sections has achromatic dispersion with an absolute value of 5 ps/nm/km or less, the influence of cumulative value of chromatic dispersion upon the whole optical fiber transmission line is small even when their lengths have fluctuated upon cutting. Alternatively, it is preferred that each of the mean transmission characteristic of the main transmission line as a whole and the substantially constant transmission characteristic of each of the first and second end sections be identical to the mean transmission characteristic (e.g., chromatic dispersion at the signal wavelength) of the optical fiber transmission line as a whole. For example, in the case where an end portion of the optical fiber transmission line is partly cut off in the process of cabling, the change in mean transmission characteristic in the optical fiber transmission line between before and after the cutting is small as long as the cutting is effected in the first and second end sections. Namely, if the main transmission line has a desirable mean transmission characteristic, then the optical fiber transmission line maintains a desirable transmission characteristic even when the first and second end sections positioned at end portions of the optical fiber transmission line are partly cut off, whereby a WDM transmission system having a transmission characteristic in conformity to its designed value can be realized if the optical fiber transmission line is employed.
Preferably, in the optical fiber transmission line according to the present invention, each of the first and second end sections has a length of 1000 m or less (5% or less with respect to the total length, for example 50 km, of optical fiber transmission line). In this case, the first or second end section can be subjected to the end portion cutting in the process of cabling, for example. Also, since each of the first and second end sections, which do not always have a transmission characteristic better than that of the main transmission line, is made shorter than the main transmission line, the favorable transmission characteristic of the whole optical fiber transmission line can be maintained.
Preferably, in the optical fiber transmission line according to the present invention, each of the first and second end sections has a mode field diameter substantially identical to that of another optical fiber to be connected thereto, such as a pigtail fiber extending from an optical amplifier disposed within a repeater station, for example. This is because of the fact that the splice loss in the junction between the optical fiber transmission line and the repeater station is kept low thereby.
Preferably, in the optical fiber transmission line according to the present invention, the mean chromatic dispersion of the main transmission line as a whole at the predetermined wavelength has an absolute value of 5 ps/nm/km or less. In this case, the mean chromatic dispersion in the main transmission line and the chromatic dispersion in each of the first and second end sections become substantially identical to each other, whereby the change in mean chromatic dispersion in the optical fiber transmission line between before and after the cutting is kept low as long as the cutting is effected in the first and second end sections. Namely, if the mean chromatic dispersion in the main transmission line is small in this optical fiber transmission line, then the state with a small mean chromatic dispersion is maintained even when the end portion cutting is effected, so that the waveform deterioration caused by nonlinear optical phenomena and cumulative chromatic dispersion is suppressed effectively, whereby a WDM transmission system having a transmission characteristic in conformity to its designed value is realized when this optical fiber transmission line is employed.
Preferably, in the optical fiber transmission line according to the present invention, the main transmission line comprises a first optical fiber having a positive chromatic dispersion at the predetermined wavelength and having an effective area of at least 40 xcexcm2, and a second optical fiber having a negative chromatic dispersion at the predetermined wavelength, whereas the first and second optical fibers are successively arranged along a signal-advancing direction. In this case, signals propagate through the first optical fiber to the second optical fiber in the main transmission line. Though the signals have a high strength when propagating through the first optical fiber having a positive chromatic dispersion, the occurrence of nonlinear optical phenomena is suppressed since the first optical fiber has a relatively large effective area of 40 xcexcm2 or greater. Since the signals lower their power while propagating through the first optical fiber, on the other hand, the signals having lowered their power propagates through the second optical fiber having a negative chromatic dispersion, thereby suppressing the occurrence of nonlinear optical phenomena in the second optical fiber as well. Further, since the whole optical fiber has a mean chromatic dispersion with a small absolute value and a small cumulative chromatic dispersion, the transmission quality is effectively restrained from deteriorating due to the cumulative chromatic dispersion.
In the optical fiber transmission line according to the present invention, the main transmission line may be a unitary optical fiber in which one or more positive dispersion portions having a positive chromatic dispersion at the predetermined wavelength and one or more negative dispersion portions having a negative chromatic dispersion at the predetermined wavelength are alternately disposed adjacent each other along the longitudinal direction thereof. In this case, signals alternately propagate through the positive and negative dispersion portions. Therefore, the occurrence of nonlinear optical phenomena (such as four-wave mixing in particular) is effectively suppressed if the absolute value of chromatic dispersion in each of the positive and negative dispersion portions is made greater. On the other hand the transmission quality is effectively restrained from deteriorating due to the cumulative chromatic dispersion if the absolute value of mean chromatic dispersion of the whole optical fiber transmission line is made smaller. Preferably, for effectively suppressing nonlinear optical phenomena, each of the positive and negative dispersion portions has a chromatic dispersion with an absolute value of 5 ps/nm/km or greater at the predetermined wavelength.
The optical cable according to the present invention comprises a plurality of transmission lines each having a structure similar to that of the optical fiber transmission line mentioned above. Here, this optical cable yields operations and effects similar to those of the above-mentioned optical fiber transmission line.
In the optical cable according to the present invention, the plurality of transmission lines comprise an upward transmission line group of N ( greater than 0) lines and a downward transmission line group of N ( greater than 0) lines having signal-advancing directions different from each other. Each of the optical fiber transmission lines included in the upstream transmission line group has a first identification marking, disposed at a portion located upstream in the signal-advancing direction thereof, for indicating that the portion is located on the upstream side and identifying the respective optical fiber transmission line, and a second identification marking, disposed at a portion located downstream in the signal-advancing direction thereof, for indicating that the portion is located on the downstream side and identifying the respective optical fiber transmission line. Each of the optical fiber transmission lines included in the downstream transmission line group has the first identification marking at a portion located upstream in the signal-advancing direction thereof, and the second identification marking disposed at a portion located downstream in the signal-advancing direction thereof.
Each of the first and second identification markings may be a colored layer disposed on a surface of the optical fiber transmission line or a mark applied to the optical fiber transmission line. When the optical fiber transmission lines included in the optical cable are thus provided with the first and second identification markings, rewinding operations in the process of connecting with repeater stations and the like become unnecessary, while facilitating the production control and restraining the operating cost from increasing. In particular, it is preferred that the first and second end sections be provided with the first and second identification markings, respectively, from the viewpoint of enhancing the efficiency in the operation of laying the optical cable.
Preferably, the optical cable according to the present invention comprises a third identification marking for indicating the directivity of each optical fiber transmission line (indicative of the signal-advancing direction), i.e., the laying direction of the optical cable. Here, the third identification marking may be a colored layer or a label applied to the optical fiber unit. In this case, optical fiber transmission lines can be distinguished from each other at each point. Preferably, a part of the optical cable excluding the optical fiber transmission lines, e.g., an optical fiber unit accommodating the optical fiber transmission lines therein, is provided with the third identification marking.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.